This invention relates to furnaces, particularly those using natural gas or other clean burning fuel gases.
Comfort and utility heating processes are widely dependent on burning combustible gases with air in a variety of furnaces. Among the prominent fuel gases are those that are essentially wholely hydrocarbon such as methane or propane. Others are mixtures of carbon monoxide and hydrogen, as such, or blended with hydrocarbon gases. Frequently these gases carry along noncombustible species such as nitrogen and water. Whatever the fuel used it is well known that conventional furnaces are rarely operated in such a way as to utilize all the potentially useful enthalpy available from the actual combustion. This is, in broadest terms, due to the fact that the gaseous combustion products are normally conducted away from the fire zone through heat exchange arrangements which extract only a part of the thermal energy so that the combustion gas residues remain at sufficiently high temperature to facilitate effective convective ejection of the exhaust gas through stacks and the like.
However, even when forced draft is used to remove and dispose of the exhaust gases, they have, until fairly recently, still rarely been deliberately cooled below the so-called dew-point (that temperature at which the concentration of water vapor is high enough to reach or exceed saturation.)
All fuel gas combustion with air results in the formation of water vapor and carbon dioxide as principal products. Depending on air-to-combustible gas feed ratio, there will be some small amount of carbon monoxide; depending on combustion temperature there will be oxides of nitrogen (designated NOx) also formed. The resulting gas must inevitably contain a large fraction of nitrogen since all normal air fed to the combustion zone will carry about 4 volumes of nitrogen for each volume of oxygen. But the air supplied to the fire is more than oxygen and nitrogen. There is always some amount of water vapor as well as small amounts of other gases (argon, CO.sub.2, transient hydrocarbons, and occasionally sulfur or halogen-bearing volatiles). The moisture in the air supply adds slightly to the moisture of the combustion gas. It is also noteworthy that the ratio of water to carbon dioxide in the combustion gas is quite dependent on the combustible gas being burned. Propane, with a higher carbon/hydrogen ratio (3:8) than methane (1:4), yields less water vapor; on the other hand, some natural gas supplies and manufactured gases inherently carry their own burden of water.
Dew-point is not the same for all fuel gas combustion processes. Besides the factors cited above it is influenced by the oxygen concentration used in converting the fuel to carbon dioxide and water. In some industrial processes, for example, feed air is occasionally enriched with raw oxygen to create a higher flame temperature. The combustion gas contains less nitrogen, and therefore a higher partial pressure of water. On the other hand one can feed excess air, resulting in a higher burden of nitrogen and therefore a lower partial pressure of water vapor in the exhaust gas. For practical purposes, however, in uses to which the present invention applies it is reasonable to expect a dew-point within a few degrees around 65.degree. C. (150.degree. F.).
Traditional furnaces embody the indirect-fired heating process; that is, the flame heat is transferred across a barrier into the heated fluid medium (air or water, as the case may be). Most of the time these systems transfer as little as 60% of the combustion heat into the heated fluid, the balance being retained in the combustion exhaust gas in order to assure its efficient disposal by thermal convection.
This waste of fuel heat value has prompted several developments. One, the so called direct-fired process, blends air to be heated with combustion gases directly without an intervening barrier. While it eliminates stack heat losses completely, the process is unsuitable for heating a stream of recirculating ambient air because of the potential of noxious gas buildup. The source of air for direct-fired heating is almost always the outdoors, which is invariably colder than the space to be heated. Thus, while the process is efficient in the sense of using all the thermal energy of the fuel, it is inefficient from the point of view of conserving heat in the space to be heated. It is most suitable for providing make up air in a space which has some other primary source of comfort heating but which suffers air losses from time to time such as in a warehouse with frequent opening and closing of doors and a fair amount of air loss to the outside. When the direct-fired process is used as the primary heating method, it is expected that continuous leakage of air to the outdoors will be in balance with the flame-heated air being brought in. By this arrangement the fraction of non-air gases is kept tolerably low.
Indirect-fired units operating under normal conditions emit approximately 50 to 200 ppm of CO (carbon monoxide), a maximum of 110 ppm of NO.sub.x, and 8000 to 10000 ppm of CO.sub.2, which is all vented to atmosphere. Direct-fired units operating under normal conditions will emit approximately 3 to 5 ppm of CO, 3 to 8 ppm of NO.sub.x, and a maximum of 2000 ppm of CO.sub.2, which is diluted by outside air as it enters the building.
Another approach to recovering more useable heat from the exhaust gas in indirect-fired combustion processes is the "high efficiency" furnace. These furnaces use two heat exchange zones: a primary zone, in which the combustion occurs, and a secondary zone, where the exhaust gases exit and cool ambient air is introduced. Exhaust gases leaving the primary zone are not removed by thermal convection as in the conventional furnace. Instead, the exhaust gases are drawn through the secondary zone by a suction fan, where they are cooled by counterflowing, incoming cool ambient air. This preheats the incoming ambient air before it enters the primary heat-exchange zone.
There are several consequences of this two-stage process. First, of course, there is a desirable effect of recovering substantially all of the sensible heat in the exhaust gas that would, in the conventional furnace, escape up a stack. But there is also the unavoidable consequent effect of creating an exhaust gas density so high that convective ejection is no longer feasible. Thus, the exhaust gas must be withdrawn and discharged from the secondary zone by means of a positive air conveyance device such as a blower or fan. Because of the cooling, however, the exhaust is also reduced in volume. These two effects (cooler and lower volume) make it possible to discharge the exhaust through smaller size ducts made of materials such as polymers which would not be suitable for the conventional furnace stacks. But another important consequence of the two-stage operation, one which is recognized as a major drawback, is that cooling of the exhaust gas to near ambient temperature in the secondary heat exchanger results inevitably in dropping the temperature of the gas below its dew-point. This causes water vapor to condense as droplets or films on the exhaust-side surfaces of the secondary zone heat exchanger. This has the desirable effect of recovering the latent heat of evaporation, but the resulting water condensate is a problem. As has already been noted, the exhaust gas contains not only nitrogen, carbon dioxide, and water vapor, but also traces of carbon monoxide and nitrogen oxides, and not infrequently also small amounts of sulfur oxides and even hydrochloric acid vapor (generated by decomposition of chlorine-bearing volatiles carried into the flame zone as contaminants of the fuel gas or combustion air). All gases are capable of dissolving to one extent or another in water. Thus, the condensed water vapor tends to absorb components from the exhaust gas to which it is exposed. Some of these components produce acidic aqueous solutions. Although the gases would dissolve only sparingly in boiling water, they dissolve more readily in the near ambient temperature which the exhaust gas is brought down to in the secondary zone. The result is creation of a highly corrosive liquid, which is the source of two serious problems. First, materials, even most grades of stainless steel, that might be used in the secondary heat exchange zone are in serious jeopardy of early failure. Second, the acidic liquid is environmentally offensive material that may be unacceptable to discharge in the sewage systems.
Another approach to recovering heat from exhaust gas is to scavenge the heat from the stack. But each of these schemes, despite variations in their design, has no effect on the primary heat transfer stage taking place in the conventional furnace fire-box. The devices are designed for and operate only to reduce the combustion heat losses occasioned by the convective ejection through stacks or exhaust gases at temperatures several hundred degrees above ambient. For example, Astle U.S Pat. No. 4,754,806, issued to the present inventor, shows a device that is very effective at removing stack heat, but it, too, is intended to work downstream of the primary heat exchanger of the conventional furnace.